Epstein-Barr virus (EBV) virion capsid antigen (VCA) gp125 as a diagnostic antigen was expressed in infected cells by recombinant vaccinia virus. The presence of IgA antibody to VCA of EBV was tested by ELISA in sera from nasopharyngeal carcinoma (NPC) patients and normal individuals. The ELISA consisted of three steps involving test serum, mouse monoclonal antibody to human IgA and rabbit anti-mouse IgG. The antibody titers of sera determined by this way correlated well with those by IF. The GMT by ELISA was 12 times higher than that by IF. Furthermore, efforts to develop gold-conjugated immunosorbent assay (GCISA), which is a more sensitive assay for measuring IgA antibody to VCA, has met with initial results.